Silly solar project No. 1: Greenhouse solar air heater

New kid on the block here, but thought I'd share the start of part 1 of my plans to warm my greenhouse using solar energy: a solar air heater (part 2 is the underground heatsink and is more likely to fail)

Inspired by some videos & photos I've seen on various places about "Build a DIY soda can heater" and the like, I'm attempting to get an air heater constructed to pull cold outdoor air through the heater and into my greenhouse in the spring & autumn (suspect it'll get warm enough in the summer on its own) to extend the growing season, and when coupled with part 2 of the grand plan should make the greenhouse a fair bit warmer even at night.

The principle is simple enough:

Build a large boxPaint it black insideRun a sealed tube or series of tubes through the box that will draw air in one end and out the otherPut glass over the top to create a sealed hotbox / greenouse style effectPut a fan at the out end to draw cold air into the tube which then gets warm in the tube as it passes through, and is then vented into the greenhouse

I'm going to be running a solar panel & battery in the greenhouse to power the fan, and the fan will be on a timer to kick in about 9am, and run for 3 hours or so.So far I've built the enclosure, done all the internals got the glass on and got it onto the allotment. Over the next few weeks I'll be connecting the heater to the greenhouse and installing the fan etc

This is the initial box - pressure treated timber sides (6") and a plywood back, with a sheet of polystyrene insulation inside to help keep the heat in. The size of the heater is 4ft x 2ft

This is the enclosure with a 80mm aluminium ducting tube threaded through it - air gets pulled in through a vent at one end, in theory heats up, and then is pulled through an out vent into pipework to the greenhouse

Everything painted black, and attached to a 90 degree bend that'll exit out of the back of the enclosure

This is with the glass attached and sealed into place, and with legs attached to put the unit at about a 25 degree tilt

And finally (for now), the installed heater in front of the greenhouse. Still need to put an extra bit of paint on the bare woodwork. But it's ready to be connected up to the greenhouse

Fascinating - what I'd like to know is the difference between incoming and exiting air temperature on a sunny day and on a dull day. You could put the warm air into a heat store pit under the house - bit like an old fashioned storage heater.

Another idea would be to collect warm air at the apex of the house and draw it down with a solar powered fan. Incidentally, large computer fans seem popular for this sort of task as they run at low voltages and are cheap.

Perhaps the effort of construction would be better invested in a solar water heater storing the warmed water in black painted barrel or tanks as a heat store, helping to keep the house warmer at night.

I'm considering building a lean-to greenhouse onto an existing stone wall which is over 60cm thick and so should provide a large thermal mass.

I remember seeing something like this being done on "its not easy being green" where they took heat from within the greenhouse and passed it over a heat sink made of crushed glass. Like many of these things there was never any feedback.

If you're trying to influence the greenhouse temperature, I would have thought it was more important to have a heat sink so that some heat would be released overnight. Certainly in my greenhouse, once bubble wrapped, the major issue is day/night temperature fluctuation. It gets warm enough during the day but the temperature drops rapidly overnight.

Last year adri123 (on this site) put butts of water in his polytunnel to behave as heat sinks (warming up during the day and giving out heat overnight). I'd thought of something similar, but have difficulty in convincing myself that the amount of heat could make a difference.

Like John, I'd be interetsed in knowing the temperature difference between the input and output.

The drawback for me with the water butts to retain solar heat and then radiate it out at night is the sheer space they take up in my mini-tunnel at the plot

This gadget would circulate heated air during the day but the main issue is the drop in temperature at night. During the day when the sun is shining it is plenty warm enough ... how would an underground heat sink work?

Funny the It's Not Easy Being Green heater idea should be mentioned as that's almost exactly what part 2 of my solar heating ideas consists of:

While I was rebuilding the greenhouse on the plot I took the opportunity to dig a large hole under where the greenhouse would be, put a recycling box in there (insulated), and filled it with broken glass, bits of rubble, mortar, and anything that would act to store the heat; then put the pipework in so it'll draw warm air in from the top of the greenhouse into the box underground, then vent it out at night. the theory being that if the solar heater warms up the greenhouse in the spring / autumn mornings, the heatsink will pull down some of the heat in spring & autumn and vent it at night to help stabilise the temperature a bit. In the summer the heatsink will pull down the heat in the greenhouse to cool it a bit, and then release it at night.

What makes it all complicated is I'm going to need 3 timers to run it all1 for the fan on the solar heater (the fan is a 140mm pc fan which is 12 volt) - to run in the mornings1 for the "in" on the heatink to pull warm air into the underground box(a 80mm PC fan in an enclosure on the pipe) - to run mid morning till late afternoon1 for the "out" fan pipe (a 40mm PC which I've bodged into the 40mm waste out pipe to pull warm air out of the box at night (and maybe help the in fan pull warm air in by creating a bit of negative air pressure)As I mentioned I got the idea from INEBG, but I also found this on youtube that gave me some ideas (and I built the box pipework as he did) 1hYxKa_CW0E (if the link doesn't work because I'm new and anti-spam and all that, add "www." to youtube.com/watch?v=1hYxKa_CW0E ) - the bloke is American and hence mangles the word cement

But more about the heat sink another time

I'm going to have to experiment with the whole heater setup to work out the temperature difference between sunny and overcast days - probably time to invest in a probe thermometer. Apparently the boxes can get up to 60 degrees on a sunny day, so if the air can get to 30 I'd be happy. if the temperature difference between sunny & overcast is significant I may have to make the rig a bit cruder and wire a 5 volt panel directly to the fan, that way it'll only turn with speed and draw warm air in when direct sunlight hits the panel (and hence the box would be hot).... In fact thinking about it I might give that a whirl first - or are there any advantages from a ventilation perspective to drawing cool air in on cool days?

I'll probably also paint a few 2 litre pop bottles black and put them in the greenhouse on the really sunny side to soak up heat on hot days then release it back out - I've only got a 8x6 greenhouse so large tanks are a no-go (though have seen videos of people using blue barrels full of water as heat stores

Either way - it's my first growing year on the plot, so it's all experimentation

What makes it all complicated is I'm going to need 3 timers to run it all1 for the fan on the solar heater (the fan is a 140mm pc fan which is 12 volt) - to run in the mornings1 for the "in" on the heatink to pull warm air into the underground box(a 80mm PC fan in an enclosure on the pipe) - to run mid morning till late afternoon

Would it be enough to hook this fan up to a solar PV panel - if there is sun it will run, if not it won't?

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1 for the "out" fan pipe (a 40mm PC which I've bodged into the 40mm waste out pipe to pull warm air out of the box at night (and maybe help the in fan pull warm air in by creating a bit of negative air pressure)

I wonder if this could be achieved with a short vertical pipe to, say, staging height. Would cold air "sink" into the store and drive the warmer air out, up the vertical pipe?

Although I've seen similar ideas in the past what I've not seen is any analysis of the systems in action. How efficient were they in terms of producing additional heat and retaining it. Remember that a greenhouse is one of the least efficient buildings you can construct due to being made of glass.

The principle of storage heaters was that the heat storage bricks went up to a high temperature and gradually released their heat over time. Will the heatstore here actually get warm enough and does it have enough capacity?

I've just been watching a fascinating DVD on constructing a large greenhouse using a mass of earth as a primary heat store: The Greenhouse of The Future